1. Field of the Invention
The present invention relates to a power amplifier, and more particularly to a power amplifier for audio apparatuses having an improved power efficiency and distortion characteristic especially in high frequency range.
2. Description of the Prior Art
There has been proposed a power amplifier which realizes high power efficiency and low distortion by combining either class-A, or class-AB or class-B amplifying circuit having a low distortion factor and a pulse-width modulation type amplifying circuit having a high power efficiency. This known power amplifier is comprised of, for example, a first amplifying circuit 3 formed with either a class-A or class-AB or class-B amplifying circuit for driving a load 2 in accordance with an input signal applied to an input terminal 1, floating power supplies 6 and 7 for supplying power supply voltages to power-amplifying transistors 4 and 5 of said first amplifying circuit 3, a second amplifying circuit 8 formed with a pulse-width modulation type amplifying circuit for driving the mid-point of said floating power supplies 6 and 7 in accordance with said input signal, and fixed power supplies 9 and 10 for supplying power supply voltages to said second amplifying circuit 8. This known power amplifier is operative so that the load 2 is driven by the first amplifying circuit 3 having a low distortion factor, and that the power supplies 6 and 7 of this first amplifying circuit 3 are controlled by the second amplifying circuit having a high power efficiency in accordance with the input signal so as to always supply a minimum level of voltage to said first amplifying circuit 3 to thereby control the transistors of said first amplifying circuit 3 so as not to saturate, thus elevating the power efficiency of the amplifier. Such power amplifier is shown in, for example, U.S. Pat. No. 4,115,739.
In order to operate this known power amplifier with a high fidelity up to a high frequency range, it is necessary that the second amplifying circuit 8 has a good high frequency range characteristic comparable to that of the first amplifying circuit 3. Otherwise, distortions in high frequency range will become extremely conspicuous. However, in order to improve the high frequency range characteristic of this known amplifier, there is the necessity for setting the carrier frequency high which determines the high frequency range characteristic of the pulse-width modulation type amplifying circuit. If this carrier frequency is to be set at, for example, several 100 kHz, there is the need for the provision of an expensive high-speed power switching element, e.g. a power MOS-FET, so that the known amplifier has the drawback that it is not suitable for practical purposes. Also, an attempt to set the carrier frequency high gives rise to problems such as an abundance of spurious radiation and switching losses.